It is often necessary in electronic circuits to attenuate a selected band of frequencies while passing a larger band of frequencies. A filter which accomplishes these purposes is commonly known as a notch filter. The use of notch filters in the cable television art is well known. A broad range of carrier frequencies is provided on a cable, each frequency corresponding to a service for which the subscriber pays. If a subscriber does not pay for selected services, it is known to place a notch filter in the coaxial line leading to the subscriber's facility to remove the frequencies corresponding to those services. If several frequencies need to be removed, the filters may be placed in series, or a single filter may be arranged to remove more than one frequency.
The passband of a notch filter is an important characteristic of the filter. With reference to the cable television arts, the passband of a filter must be large enough to accommodate the range of frequencies supplied by the cable television service. Thus, if the high-frequency roll-off of a filter is too great, the attention of the high frequencies makes them unusable. The frequency range of the cable television band was one time 55-300 MHz. The upper range was subsequently raised to 400 MHz, and it has now been raised again to 600 MHz. Accordingly, the passband of notch filters used in this art is critical.
FIG. 1 shows a schematic circuit diagram of a theoretical notch filter. An inductive-capacitive network 2 comprising capacitors C2, inductor L1, and capacitor 7 is placed in series with input signal 4 and input resistor 5 to remove selected frequencies. An inductor 9 couples removed frequencies to ground, and the output appears across output resistor R0. The notch produced by the filter shown in FIG. 1 is generally symmetrical, and this circuit can theoretically achieve an infinitely deep notch if certain conditions are met. The circuit, however, suffers from the undesirable limitation that the component ranges are critical for notch widths less than ten percent of the notch frequency. This makes the circuit shown in FIG. 1 wholly impractical.
FIG. 2 is a schematic diagram of a practical compromise circuit based on the theoretical circuit shown in FIG. 1. In this circuit, an inductor 6 and a capacitor 8 have been added to the network 2. The addition of these components permits the use of practical element values having ranges which are more realistic. FIG. 2a illustrates the equivalent notch performance of the circuit shown in FIG. 2, the component values there being combinations of the values of the components shown in FIG. 2 and being indicated by primed reference numerals.
The circuit shown in FIG. 2 achieved great flexibility in notch filter design, but it suffers from undesirable tradeoffs. FIG. 3 is a high-frequency circuit model of the circuit shown in FIG. 2. Because of the presence of capacitor 8 and inductor 6, added to achieve reasonable component parameters, the maximum pass frequency is limited.